Loud-speaking and amplified microphone telephone exchange



Dec. 21, 1937. E; LEVEQUE 2,102,903

LOUD SPEAKING AND AMPLII 'IED MICROPHONE TELEPHONE EXCHANGE Filed Dec. 27, 1934 2 Sheets-Sheet l I 4 Fig.2.

Ewu'Ze Leveg'uc Inverzfor:

XWMA D ec. 21, 1937. E. LEVEQUE LOUD SPEAKING AND AMPLIFIED MICROPHONE TELEPHONE EXCHANGE Filed Dec. 27, 1954 2 Sheets-Sheet 2 H I 7; van 2 0 H Patented Dec. '21, 1937 UNITED STATES UD-SPEAK ING AND AMPLIFIED MICRO- PHONE TELEPHONE EXCHANGE Emile Iieveque, Iaris, France, assignor to firm Le Telephone Prive National Socit Anonyme, Paris, France, a corporation or France Application December 27, 1934, Serial No. 759,407 In France December 30, 1933 9 Claims. (01. 179-4) Telephone systems are known which comprise a loud speaker and an amplified microphone permitting the subscribers to hear each other without taking the receiver to their ears and to speak di- 5 rectly into a microphone. It is also known to have relays in such systems which are controlled by the voice of the speaker,'said relays controlling certain circuitsof the system so that an acoustic coupling between the microphone and the loud 10 speaker of the same station cannot. take place. These vocally controlled relays are generally of the differential type so that the direction of the paratus provided with a-micro-receiver. Indeed in these latter devices the danger of coupling does 25 not exist and, as a directing relay is absent, the sensitivity of the device is greatly increased. It may be said that the two subscribers feel each other. It is not so in the loud-speaking and amplified microphone telephone systems where 30 the subscribers having nothing to fix their attention, are inclined to answer either too early or too late. They cannot follow the varieties of the conversation so well. They are less independent of what takes place around them because they have noreceiver on the ears.

The present invention aims to provide a system where the various difliculties arising between the attendants of the two stations equipped with loud speaker and amplified microphone are lessened 40 so much that the training which had to be acquired hitherto by long practice is automaticab ly regulated. For this reason such systems may be left in the hands of any body and their use will be more extensive than has been the .case

up to the present.

The system according to the present invention is constructed in a manner that each station is equipped so that it can take the initiative of a.

conversation independently of the direction which 50 might be given vocally. To this feature may be added that of a signalling system advising the attendants of the corresponding stations oi the direction of the conversation and of the condition of the circuits 1! open or engaged. These signals enable the attendants:

(a) to concentrate their attention in the direction of the transmitter,

'(b) to control the establishing of the connection at the moment of the call;

to. make sure that one of theattendants may speak so that the attendant at the other station may hear him,

(d) or, on the contrary, that one attendant should not speak because the attendant at the other station could not hear him,

(e) to advise them to speak sumclently loudthat they may be well understood,

(f) to advise them that the connection is actually interrupted when the conversation has ended. v i The different named features-may be combined in certain cases with each-other or be partly lacking. The means for producing signals mentioned above may comprise any known form.

In the preferred form of the invention a certain number 0! stations having loud speakers and amplified microphones are grouped in an automatic central exchange .comprising as is usual intercommunicating groups by means of which the attendants may be connected automatically with each other. In such a case the amplifying devices -.for the loud speakers as well as those for the microphones are connected to the central exchange where they belong tothese intercommunicating groups. Also means may be provided for signalling from one station to the. other in spite of the automatic operation, in the same manner as if the two stations were connected without the intermediary of a central exchange.

The annexed drawings show as examples three diagrams of systems according to the present invention. a

Fig. 1 is a diagram showing two stations connected directly and provided with means for reciprocal intervention for directing the conversa- 40 tion.

two stations may be connected, said stations being provided with signalling systems similar to those of Fig. 2.

with reference to Fig. 1 the stations I and II are each equipped with a loud speaker HP and a very sensitive microphone M. Besides they include a calling button a and. an intervention button i. At each station there isa buzzer R. The outfit common to both stations comprises two fiers are of a known system and shall not be described.

For establishing communication between station I and II the attendant at station I presses the button a and connects the buzzer with thepositive poleof a battery and with the amplifiers through the contacts A--l and A.3, said amplifiers transmitting the signal of the buzzer to the respective loud speaker. This signal passes to the microphone of station I and the produced current acts on the relay C.

A weakening resistance of preventing the acoustic coupling between the microphone and speaker of the same station is inserted in each of the loud speaker currents; If the attendant at station I speaks, the associated relay C is energized to open contact C--|, thus interrupting the weakening circuit, and to open contact C2 to break the circuit of the loud speaker at station I. Thus, the microphone of the hearer at station II is rendered ineifective.

This latter, therefore, must wait until the attendant at station I has stopped speaking before he may speak. It may be necessary, however, to

interrupt before. This is done by means of but-' ton i at station II which allows the attendant at this station to switch on his transmitter into the circuit of the loud speaker of station I. This is effected by closing a circuit in parallel with contact C-2 of the relay C and by the instant reestablishing of another weakening circuit including a resistance a). which is also connected to said intervention button 1'.

When the conversation is to be ended the attendant at the station speaking presses the button d to reestablish the initial position of all parts. ,How this is done is not represented because it has nothing to do with the present invention.

With reference to Fig. 2 of the drawings, the stations I and II are likewise provided with a loud speaker HP, a microphone M, a calling button a, an intervention button 1 and a button d for ending the conversation. Each station possesses, in addition, the signalling lamps V (green), B (white), R (red) or V1, B1, R1 respectively.

The two stations are shown directly connected but they could also be connected intermediately by an automatic device of known construction.

weakening circuits, including resistances a are provided according to need by the control relays C, or C having each contacts I, 2 and 3 and a buzzer. R for calling. 1 When the operator at station I wants to establish connection with 11 he presses the button a and the amplifiers AM1 and AMz are inserted into the circuit. The buzzer R is operated, .but placed after the amplifier. There is no relay ,0 of the station I. By way of compensation,

however, the noise produced in the loud speaker HP of the station II is retransmitted by the microphone of station II. It results by this that relay C is energized which at its contact 2 lights the green lamp V of the called station II and the red lamp R of the calling station I. The operator at station I therefore has control of his call and the called operator is invited to make himself known. If this has been done the relay C will drop again and the red lamp of station I and the green lamp of station 11 will be extinguished. However the two white lamps B and B will be lit to prove that the connection has been established and that the line is free.

As soon as one of the operators'begins to speak the relay C of station I or the relay C of the station II is energized and whatever be the direction of the conversation it is the green lamp of the caller and the red lamp of the called which are lit alternatively.

The relay C or 0' not only directs the conversation in one way or the other by cutting or reestablishing the circuit including the microphone of one of the operators and the loud speaker of the other operator, but also advises them of the direction of the conversation if the line is free.

In this form of the invention there is also an intervention button 2' by means of which one of the operators can shortcircuit the cut contact of his microphone and of the loud speaker of the other station in order to take the initiative of the conversation. It is not necessary to describe the working of this button for it is the same as in the case of Fig. 1.

In Fig. 3 an intercommunicating group is represented between a central exchange and an autoswitch. This group is equipped with intervention and signalling devices described in the above. The station I is supposed to be the caller and the station II the called. It is clear that these characters may be reversed. The part III of the drawings is the central exchange- The connection is established as follows:

The caller switches his station to the central exchange by pressing the calling button. When the connection has been made which is marked .optically the attendant connects himself with the desired station by means of his finger dial.

reason, he sends an auditory signal in the loud a speaking set of the caller by means of a refusing button, making use for instant of the calling circuits. Notified by this signal the caller cuts the communication.

When the called operator accepts the communication, he announces himself. Then the unilateral communication, caller towards called person may continue.

If, during the conversation, one of the operators wants to interrupt the other and to take for a moment the priority of the conversation, he presses on an interfering button acting thus on the relays which invert the priority to his advantage.

'Finally, if one of the operators, after having obtained the priority of the conversation wants to stabilize the communication in one direction only, he presses on his calling button. When he has to dictate a letter, to give a lecture or to transmit a fairly long communication, he thus avoids that at each temporary cessation in speaking, the relay system in the central exchange begins to work by taking ofi his priority of conversation said relay reestablishing the connection as soon as he begins to speak again.

arcades This operation does not, however, prevent the :alled operator from interrupting and thus rernoving the unilateral stabilization of the comapparatuses generally used 'in automatic telephany, such as a call finder CA, a line selector SU, an

impulse sender EI and diiferent relays belongingtoeach intercornmunicating group.

The exchange comprises for each intercomrnunicating group symmetrically arranged an amplifier L1, L3, a rectifier valve La and L4 respectively, the transformers Trl, 'Ir2 and Tr3, Tr respectively.

The intercommunicatlng group comprises the locking relays C; C, control relays H, H, interfering relays IN, IN, stabilizing relays F, F, feeding relays D, D','D", a group relay 1, coupling relays UU, impulse relays Y, call relays 1313'; each station possesses a change over relay for the signalling lamps S, S and a line relay AA".-

The working principle of. the telephone ex-' change is as follows: v

1. Calling.-The caller presses on the button a. In the exchange the light armature I of his line relay A as well as the armature of the impulse relay-Y are energized.

Circuit: negative-connection VIII-contact I of the button a-connection VII-winding I of relay A--back contact A4--back contact on the fuse carrier Pf-back contact Baa-winding I of relay Y-positive.

The contact A: prepares the circuit of test, contact Y3 prepares the circuit of the group relay T--contact Y1 energizes the call finder CA.

Circuit: negative' winding'of .the call finder CA-working contact Y1-.-back contact Bz-b'ack contact T1impulse sender EI-positive. The call finder rotates by going on the line of the caller, the armature of the relay T and the heavy armature II of the relay A are attracted.

Contact T1 cuts off the call finder-CA which stops on the line of the caller and energizes the relay 3. Contact A: cuts the short-circuit of the line III-IVcontact A lights the red lamp on the calling station, thus advising the attendant that the central exchange is ready to be dialed.

Circuit: negative-contact A1 connection IX- vback contact Sa-red lamp R-connection X-- From the first impulse the contact Y: energizes the relay J and the line selector SU.- The latter makes a step at each impulse; the retarded relay J holds itself during the continuance of the impulse train.

When the relay J is energized, the contact J1 cuts the short-circuit of the winding I of the re-.

lay Y-contact J'z shunts the retarded windin of J and holds the relay U short-circuited.-,

Contact J3 cuts the circuit of the relay T. The selector SU'being out of its position of rest the contact. SUI prepares the setting to rest of they selector SU. Contact SU: cuts oil? a short-circuit of the relay U and prepares a short-circuit -of the winding I of relay Y.

When the dialing is finished, the relay J drops. Contact J2 opens a short-circuit .of the relay U which is energized, contact Ja completes the cir cult of the relay T.

The relay U being energized, the contacts U1, U3 separate the line III-IV from the relay Y and connect it to the transformer Tr 4, contact Us cuts the numbering circuit. The contact U4 prepares a circuit of the relay NR; contact U5 prepares a holding circuit of the relays D+-D'-- D". The contact Us holds the'relay B.

The relay Y drops.

3. (a) The called station is busy-The relay T is short-circuited by the winding of small resistance of the relays T orthe relay U of the group by which this station is busy. At the calling station the red lamp remains lit, no noise is perceived in the loud speaker HP. The caller is thus advised that the called station is not free.

(b) The called station is free-At the dropping of the armature of the relay J the relay Tr of the group and the relay A of the station are attracted.

Circuit: positive-B5 working-J: back-winding 11-1 of relay Tarm C of the selector SU-- winding II of relay A--connecticn VIrest of button al -connection VIII-negative. If the relay T is energized the contact T2 cuts the circuit of the calling relay Ethe contact T3 cuts a circuit of relay NR.the contact T4 energizes the relays E--D-D-D"--The contact T5 cuts the dial circuit-The contact Te short-circuits the winding II of the relay T. The relays E--D'D" are energized simultaneously but the hanging of E is sensibly quicker.

By energizing of relay E the contact E1 shortcircuits the relay of group G. The contact E:

energizes a buzzer R0 in series with the windings I and II of the transformer Trl-the contact E3 short-circuits the resistance RC placed between the windings of the transformer TrZ-The contact E4 closes a holding of the relay EEe shortcircuits the relay' G1 by a resistance.

The relays DD'-'-D" energized: The con tacts D1.and D4 prepare a circuit of the calling microphone. The contacts D: and D5 prepare a circuit of the called microphone.-The contact .Ds closes the circuit of the filament of the 4 triocles L1 L2 L3 L4D1 prepares the receiving circuit of the called-The contact D's prepares the circuit of the change over relay S of the signalling lamps of the station II--D"1 prepares the receiving circuit'of the called-The contact D": cuts the energizing circuit of the calling relay E and maintains the relays E--D--D"--D. The contact D"4 prepares the circuit of the control relays H and H'Ds completes the circuit of the change over relay of the lamps S of the calling station-The contacts Si. and S2 light up the white lamp, and the red lamp extinguishes.

The contacts D; and D"; shall prevent that.

during the rest of the intercominunication group numbers no current be inducted within the circuits of the loud speakers of the stations, currents which could produce a call.

4. Call signal.When the contact 1B: is closed the called receives in his loud speaker an oscillation transmitted to him by the buzzer R0.

. Circuit: positivebuzzer -R0E2 workingprimary winding I of the transformer Tr lwinding 1 of the induction coilnegative.

The oscillating current passing the winding I of the transformer Tr I is transmitted to the grid of the triode L1 which produces a current of the same nature, but amplified in its anode circuit in which is included also the primary of the transformer Tr 2. 7

Circuit: negative common with the batteries (central and anodes), filament and space of filament and plate of the lamp L1-back contact of relay NR1-winding I of the transformer Tr 2 contact C'1--positive anodes.

The currents produced in the windings II and III of the transformer Tr 2 are transmitted to the called loud speaker by passing through the selector SU, arm d, e and the calling plate DA.

The oscillations received by the called loud speaker are retransmitted by acoustic coupling to the central exchange by the microphone at the called station.

Circuit: winding II of the transformer Tr 3 working contact Da-arm a of the selector SU- connection II-working contact Ss-back contact of button in'-microphone M'connection 1-- arm b of SUworking contact D5-winding I of the transformer Tr 3--condenser C3; the microphone being fed across the induction coil S2.

These microphonic currents are transmitted to the grid of the amplifying tube L3 by the intermediate of the winding III of the transformer Tr 3, they produce currents of the same nature, but amplified in the anode circuit of L3 in which is included the primary winding I of the transformer Tr 4.

Circuit: negative common to the batteries (central and anodes) filament and space of filament and plate of L3back-contact NRa--winding I of the transformer Tr 4-back contact C1 positive anode.

The currents of the winding I of the transformer 4 are transmitted, one part to the calling loud speaker by the intermediate of the windings II and III of the Tr 4. The caller is thus advised that the called has been called.

On the other part these currents arrive at the grid of the triode L4 by the intermediate of the winding IV of the transformer Tr 4. They produce an amplified and detected current in the anode circuit of. L4 in which the winding of relay G' is found.

The intensity of the current increases until it suflices-to cause the attracting of the armature of' relay G. At this moment the contact G's relieves the short circuiting of the winding of the relay Hf which is energized.

When the relay H is energized the contact H'1 cuts the calling relay E which drops and cuts the call, the contact H's energizes the relay 0'.

By energizing the relay C the contact Ci cuts the anode circuit of the amplifying lamp L1. The contact C'2 relieves the short-circuiting of a resistance RS placed in the signalling circuit of the called. Therefrom results the dropping of the heavy armature I of his relay S and the lighting of his green lamp by the working contact S: and back contact S'1The contact 0:

short circuits the relay GC4 short-circuits the receiver line III-IV of the called. The contact C's cuts the signalling circuit of the caller. His relay S drops and causes the extinguishing of his lamp and the lighting of his red lamp by the back contact S2. The contact C's short circuits the relay C during the different commutations caused by the relay C. The contact C'a short-circuits the weakening resistance RC placed between II and III of the transformer 4.

5. (a) Being busy the called refuses the conversation Having received the call, the operator presses his refusing button d. At the central exchange his connecting relay A and the group relay T drop. The calling relay E is again energized.

Circuit: positive-U5 workingDz working- T2 back-winding of E, negative.

At the dropping of relay T the contact Ta closes the circuit of the relay NR which is energized.

Circuit: 'positive-D5 working-T3 back-U4 workingwinding of NR--negative.

The relay E controls again the calling circuit. The relay NR reverses at its contacts NR1 and NR: the anode circuits of the triodes L1 and La and the oscillating current produced by the buzzer R0 and amplified by the triode L1 will be received this time by the intermediate of the transformer Tr 4 by the loud speaker of the caller.

Advised by this signal the caller presses his button d whereby the connection made will be cut off at the central exchange.

himself known by the words here station X.

This answer is transmitted by his amplified 'microphone by L3, is received by the loud speaker of the caller and by the detecting tube L; which holds the relay G by the circuits following in the explanation given for the working of the call.

When the called has answered he leaves the speech to the caller. As the relay G receives no more current it will drop, the control relay H and the locking relay will drop retarded. The contact Ci reestablishes the anode circuit of L1. The contact C'z short-circuits the resistance RS included in the signalling circuit of the called which will cause the energization of the heavy armature II of his change over relay of the lamps S and will cause the lighting of the white lamp by the working contacts S'i and Sz.

The contact C's short-circuits the relay G and the contact 0'4 short-circuits the receiver line III-IV of the called. The contact C's closes again the signalling circuit of the caller which will cause the energizing of his relay S and will cause the lighting of the white lamp by the working contacts S1 and S2. The contact C's short-circuits the weakening resistance RE placed between the windings II and III of the transformer Tr 4.

At this moment the white lamp at the two stations advises that the double connection (going and returning) has been established. The eflect of the receiving is moderated by the resistances placed between 4 and 5 of the transformers Tr 2, Tr 4 and by the condensers shunted with the loud speakers by the working contacts S3. This efiect is regulated slightly above the limit of priming.

' called loud speaker, the detectlnsb lamp L1 and the relay G which is energized.

When this relay G is energized the cohtactGr blocks the relay G'by short circuiting the same. The contact (is short-circuits the control relay H which is energized. The contact H1 energizes the locking relay C and light armature I of the stabilizing relay ,F.. The locking relay being energized, the contact C1 cuts the anode circuit of La. Contact Ca opens the circuit of the resistanceRS included into the signalling'circuit of the calling station. the result of which is to cause. the dropping of the heavy armature II of its relay S and to cause the lighting of itsgreen lamp by means of the working contact 8: and the back contact 81,

.The contact C4 short-circuits the receiving line of the caller. Contact Cs cuts the signalling circuit of the called station whereupon the relay called station) presses once his button in.

var

S is dropped and its red lamp is lighted by the back contact S'a. The contact Cc short-circuits the relay G during the process of commutation assured by the locking relay C. The contact Cs'ShOI't-Oll'fllltfi the weakening resist-' ance RC connected between. the windings'II and IIIoi the transformerTrL v 'The contact I: prepares the circuit of the control-winding oithe heavy armature stabilizing relay F. Y On this principle the communication may continue without limit. Each. time that one of the interlocutors stops to speak the communication becomes bilateral (white lamp at the two stations) and the initiative of the conversation may be taken up by one. or the other of the attendants.

'7.-'-Interferina.--However if one ofthe two operators is leading'theconversation, it may be useful that the other he ableto interrupt him. In this case, the. latter (the operator on the His microphone is then cutout by the contact I and the interfering relay IN is energized by the contact 2 of the button in. Y f

Circuit: positive-connection X-'-contact 2 of the button in'-connection J -'-arm f of selector SU-back contact C's-working contact C the relay INnegative.

The interfering relay IN being energized, the contact IN; cuts the circuit of the central relay H-The contact IN'g'cutsthe circuit of the relay G. Contact IN'a cuts the retarded circuit of the 'relay H, 'so that the latter may drop rapidly. The contact INs completes a, holding circuit of the relay IN.

Circuit: positive-working contact B5wo rkin'g contact IN'sback contact H's-winding oi the relay IN'negative..- v

The relay H being cut oh, its contact H1 opens the circuit of the locking relay C which drops. The contact Cr completes the anode circuit of the amplifier L3. Contact C4 opens the short-circuit of the loud speaker line of the caller. By thi fact, the initiative of the conversation is given to the attendant at the called station. On the other hand, when the latter. lets go the interfering button in, his microphone circuit is com leted again. The current impulse generated thereby energizes the relay G: The contact G's opens theshort-circuit of the winding I of control relay H, the armature of which is consequently attracted. The contact Hz energizes the locking relay C which, by means of its contact (2' shortlIoi'the.

, S circuits the loud speaker line of the attendant at the called station. The contact C'1 opens the anode circuit of the amplifier L1, so that microphone currents of the station I are not transmitted to the loud speaker circuit of the called station. The caller loses his priority. The contact H: cuts out the relay IN which drops, its

function being terminated.

The caller may, in his turn, interfere in the same way. This time it will be the relay IN which to his advantage will play the part which the relay IN has played to the advantage of the caled station. The buttons in and in shall also be used each time when there is noise in the-room where the called interlocutor is staying.

8.Stabilizina the communication in one direction Ii one of the interlocutors having the initiative 0,1. the conversation wants to dictate a letter, to

' lecture or to make a fairly long conversation, he

may stabilize the communication in the direction in which he is speaking by pressing on his button a. He thus energizes the stabilizing relay F in thecase of the caller, or the relay F respectively for the calling station.

Circuit: (for the caller) positive-connection X -contact 2 of the button a-connection IV- working contact A3-arm d of. the call finder CA remains then stabilized in the direction from the speaker to the bearer. However, the bearer always retains the possibility of interrupting by pressing once the button in. By this operation the relay IN is energized and the contact INa cuts the holding circuit of the; relay F. Everything becomes normal again.

What Iclaim as new is:

1. A telephone system comprising a plurality of stations, each including a transmitter and a loudspeaking receiver, a circuit including an amplifier connecting the transmitter at one station with the receiver at a second station, a second circuit ineluding an amplifier connecting the transmitter 'at the second station with the receiver at the first circuit will be opened to break the last-recited circuit, a manually-controlled interrupting switch at the station including the transmitter of the flrsti'ecited circuit, a circuit including said interrupting switch and shunted across the switch in saidfi-rst-recited circuit, a second-manually-controlled interrupting switch at the second station, and a. circuit including said second interrupting switch and shunted across the switch in said second circuit.

2. A telephone system comprising a plurality of stations, each including a transmitter and a loudspeaking receiver, a circuit including an amplifier connecting the transmitter at one station with the receiver at a second station, a second circuit ineluding an amplifier connecting the transmitter at the second station with the receiver at the first station, a relay, a relay-opened switch in each cir-. cuit, the relay in one circuit controlling the switch in the other circuit, whereby, upon energizing the relay in one circuit, the switch in the other circuit will be opened to break the last-recited circuit, a circuit including a resistance and a switch shunted across each receiver, and means controlled by the relay in each of the receiver circuits upon energization of the respective relays for opening the switch to break the respective associated resistance containing circuit.

3. A telephone system comprising a plurality of stations, each including a transmitter and a loudspeaking receiver, a circuit including an amplifier connecting the transmitterat one station with the receiver at a second station, a second circuit including an, amplifier connecting the transmitter at the second station with the receiver at the first station, a relay, a relay-opened switch in each circuit, the relay in one circuit controlling the switch in the other circuit, whereby, upon energizing the relay in one circuit, the switch' in the other circuit will be opened to break'the last-recited circuit, a circuit including a resistance and a switch shunted across each receiver, means controlled by the relay in each of the receiver circuits upon energization of the respective relays for opening the switch to break the respective associated resistance containing circuit, a manually-controlled interrupting switch at the station including the transmitter of the first-recited circuit, a circuit including said interrupting switch and shunted across the switch in said first-recited circuit, a second manually-controlled interrupting switch at the second station, a circuit including said second interrupting switch and shunted across the switch in said second circuit, and a circuit including a resistance associated with the interrupting switch at one station for shunting the receiver at the same station upon actuation of the interrupting switch.

4. A telephone system comprising a plurality of stations, each including a transmitter and a loudspeaking receiver, a circuit including an amplifier connecting the transmitter at one station with the receiver at a second station, a second circuit including an amplifier connecting the transmitter at the second station with the receiver at the first station, a relay, a relay-opened switch in each circuit, the relay in one circuit controlling the switch in the other circuit, whereby, upon energizing the relay in one circuit, the switch in the other circuit will be opened to break the lastrecited circuit, a pair of signal lights of the same color, one at each station, a circuit connecting said lights in parallel, a two-way switch associated with each relay, a circuit serially including said two-way switches for energizing said parallel light circuit when both two-way switches are unaffected by the relays, a light of a second color and a light of a third color at each station, a circuit connecting the light of the second color at one station with the light of the third color at the other station, and means including said twoway relay-actuated switches for connecting said light circuits-to cause the light of the second color to be lighted at the speaking station and the light of the third color to be lighted at the listening station during transmission.

5. A telephone system comprising a plurality of stations, each including a transmitter and a loudspeaking receiver, a circuit including an amplifier connecting the transmitter at one station with the receiver at a second station, a second circuit including an amplifier connecting the transmitter at the second station with the receiver at the first station, a relay, a relay-opened switch in each circuit, the relay in one circuit controlling the switch in the other circuit, whereby, upon energizing the relay in one circuit, the switch in the other circuit will be opened to break the lastrecited circuit, an interrupting switchat the station including the transmitter of the first-recited Y circuit, a circuit including said interrupting switch and shunted across the switch in said first-recited circuit, a second interrupting switch at the second station, a circuit including said second interrupt ing switch and shunted across the switch in said second circuit, a pair of signal lights of the same color, one at each station, a circuit connecting said lights in parallel, a two-way switch associated with each relay, a circuit serially including said two-way switches for energizing said parallel light circuit when both two-way switches are unaffected by the relays, a light of a second color and a light of a third color at each station, a circuit connecting the light of the second color at one station with the light of the third color at the other station, and means including said two-way relay-actuated switches for connecting said light circuits to cause the light of the second color to be lighted at the speaking station and the light of the third color ,to be lighted at the listening stai tion during transmission.

6. A telephone system comprising a plurality of stations, each including a transmitter, a loudspeaking receiver and a station calling dial, a central exchange common to said stations, a circuit at said exchange for transmitting from the calling station to the called station, a second circuit at the exchange for transmitting from the called to the calling station, means at the exchange operable upon transmission over one circuit for preventing transmission over the other circuit, a call finder at the exchange for connecting said circuits to the calling station, means at the calling station and at the exchange for actuating said call finder, a line selector for connecting said circuits to the called station, means actuated by the dial at the calling station for operating said line selector, interrupting means at the exchange for blocking transmission over either of said circuits, and means at said stations and connected through said call finder and line selector to said interrupting means for controlling the latter.

7. A telephone system comprising a plurality I of stations, each including a transmitter, a loudspeaking receiver and a station calling dial, a central exchange common to said stations, a circuit at said exchange for transmitting from the calling station to the called station, a second circuit at the exchange for transmitting from the called to the calling station, means at the exchange operable upon transmission over one circuit for preventing transmission over the other circuit, a call finder at the exchange for connecting said circuits to the calling station, means at thecalling station and at the exchange for actuating said call finder, a line selector for connecting said circuits to the called station, means actuated by the dial at the calling station for operating said line selector, signal lamps at each gizing alight of a second color at the calling station and a light of a third color at the called station, and means operable upon transmission from the called to the calling station for energizing a light of the second color at the called station and a light of the third color at the calling station.

8. A telephone system comprising a plurality of stations, each including a transmitter, a loud speaking receiver and a station calling dial, a central exchange common to said stations, a circuit at said exchange for transmitting from the calling station to the called station, a second circuit at the exchange for transmitting from the called to the calling station, means at the exchange operable upon transmission over one circuit for preventing transmission over the other circuit, a call finder at the exchange forconnecting said circuits to the calling station, means at the calling station and at the exchange for actuating said call finder, a line selector for connecting said circuits to the called station, means actuated by the dial at the calling station for operating said line selector, a calling contactor at each station, and stabilizing means at the exchangeunder control of the calling contactors blocked upon a pause for maintaining one of said transmission circuits the other circuit.-

9. A telephone system comprisinga plurality of stations, each including a transmitter, a loudspeaking receiver and a station calling. dial, a central exchange common to said stations, a circuit at said exchange for transmitting from the calling station to the called station, a second circuit at the exchange for transmitting from the called to the calling station, means at the exchange operable upon transmission over one circuit for preventing transmission over the other circuit, a call finder at the exchange for connecting said circuits to the calling station, means at the calling station and at the exchange for actuating said call finder, a line selector for connecting said circuits to the called station, means actuated by the dial at the calling station for operating said line selector, a refusing contactor at each station, and means at the exchange controlled by the refusing contactor for causing a refusing signal to be transmitted from the called station to the calling station.

EMILE LEVEQUE.

in the transmission over 

